Place fonts in the mod's font folder.
%user%\AppData\LocalLow\Owlcat Games\Warhammer 40000 Rogue Trader\UnityModManager\FontMod\Fonts. (WrathInstallDir)\Mods\FontMod\Fonts (KingmakerInstallDir)\Mods\FontMod\Fonts
Ctrl+F10 to open UMM menu to change mappings
default mapping will change ALL in-game fonts unless otherwise set. This is the first font loaded on first run if fontMappings.json does not exist.
M button next to in-game font will let you map an installed font to it. I button next to in-game font will ignore. X button next to in-game font will remove mapping.
After mapping the next time the scene loads the fonts will be updated.
I don't feel like mining all the different fonts used by the game. New font names discovered will placed in encounteredFonts.json. Map as you see fit. If you know the font name it can just added manually to fontMappings.json.